Memory Management Paging (continued) Segmentation CS350 & CS552

Implementation Issues in Paging Four times when OS involved with paging Process creation determine program’s resident-set size create page table Process execution MMU reset for new process TLB flushed Page fault time determine virtual address causing fault swap target page out, needed page in Process termination time release page table, pages CS552

Page Fault Handling (1) Hardware traps to kernel, t1 is interrupted Context switch into kernel: Privilege mode ON, Interrupts OFF, Protection OFF General (users) registers saved OS determines which virtual page needed OS checks validity of virtual address OS Seeks target/victim; if victim dirty, then write it to disk OS schedules I/O request for new page from disk OS dispatches another task (tn), while waiting Page-in I/O completes, t2 registers saved Page tables updated Faulting instruction backed up to when it began Faulting process scheduled Registers restored Program (t1) continues CS552

Locking Pages in Memory Virtual memory and I/O occasionally interact Proc issues call for read from device into buffer while waiting for I/O, another processes starts up has a page fault buffer for the first proc may be chosen to be paged out Need to specify some pages locked exempted from being target pages CS552

Separation of Policy and Mechanism Page fault handling with an external pager CS552

Segmentation (Pt1) One-dimensional address space with growing tables One table may bump into another (solution?) CS552

Allows each table to grow or shrink, independently Segmentation (Pt2) Allows each table to grow or shrink, independently CS552

Comparison of paging and segmentation Segmentation (Pt3) Comparison of paging and segmentation CS552

Implementation of Pure Segmentation (a)-(d) Development of checkerboarding (external fragmentation) (e) Removal of the checkerboarding by compaction CS552

Segmentation with Paging: MULTICS (Pt1) Segment descriptor – numbers are field lengths Descriptor segment points to page tables CS552

Segmentation with Paging: MULTICS (Pt2) Conversion of a 2-part MULTICS address into a main memory address CS552

Segmentation with Paging: Pentium (Pt1) Similar to MULTICS, but addresses a number of different design goals A Pentium selector GDT = Global Descriptor Table LDT = Local Descriptor Table Code segment descriptor Data segments differ slightly CS552

Segmentation with Paging: Pentium (Pt2) Conversion of a (selector, offset) pair to a linear address CS552

Segmentation with Paging: Pentium (Pt3) Mapping of a linear address onto a physical address CS552

Segmentation with Paging: Pentium (Pt4) Level Protection on the Pentium CS552

References Chapters 8 and 9 :OS Concepts, Silberschatz, Galvin, Gagne Chapter 4: Modern Operating Systems, Andrew S. Tanenbaum X86 architecture http://en.wikipedia.org/wiki/Memory_segment Memory segment http://en.wikipedia.org/wiki/X86 Memory model http://en.wikipedia.org/wiki/Memory_model IA-32 Intel Architecture Software Developer’s Manual, Volume 1: Basic Architecture http://www.intel.com/design/pentium4/manuals/index_new.htm CS552